[tip:x86/asm] x86/asm/decoder: Fix and enforce max instruction size in the insn decoder

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  91e5ed49fca09c2b83b262b9757d1376ee2b46c3
Gitweb:     http://git.kernel.org/tip/91e5ed49fca09c2b83b262b9757d1376ee2b46c3
Author:     Andy Lutomirski <luto@amacapital.net>
AuthorDate: Tue, 27 Jan 2015 16:06:02 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 19 Feb 2015 00:01:24 +0100

x86/asm/decoder: Fix and enforce max instruction size in the insn decoder

x86 instructions cannot exceed 15 bytes, and the instruction
decoder should enforce that.  Prior to 6ba48ff46f76, the
instruction length limit was implicitly set to 16, which was an
approximation of 15, but there is currently no limit at all.

Fix MAX_INSN_SIZE (it should be 15, not 16), and fix the decoder
to reject instructions that exceed MAX_INSN_SIZE.

Other than potentially confusing some of the decoder sanity
checks, I'm not aware of any actual problems that omitting this
check would cause, nor am I aware of any practical problems
caused by the MAX_INSN_SIZE error.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Fixes: 6ba48ff46f76 ("x86: Remove arbitrary instruction size limit ...
Link: http://lkml.kernel.org/r/f8f0bc9b8c58cfd6830f7d88400bf1396cbdcd0f.1422403511.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/insn.h | 2 +-
 arch/x86/lib/insn.c         | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
index 47f29b1..e7814b7 100644
--- a/arch/x86/include/asm/insn.h
+++ b/arch/x86/include/asm/insn.h
@@ -69,7 +69,7 @@ struct insn {
 	const insn_byte_t *next_byte;
 };
 
-#define MAX_INSN_SIZE	16
+#define MAX_INSN_SIZE	15
 
 #define X86_MODRM_MOD(modrm) (((modrm) & 0xc0) >> 6)
 #define X86_MODRM_REG(modrm) (((modrm) & 0x38) >> 3)
diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 5ea00dd..8f72b33 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -52,6 +52,13 @@
  */
 void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
 {
+	/*
+	 * Instructions longer than MAX_INSN_SIZE (15 bytes) are invalid
+	 * even if the input buffer is long enough to hold them.
+	 */
+	if (buf_len > MAX_INSN_SIZE)
+		buf_len = MAX_INSN_SIZE;
+
 	memset(insn, 0, sizeof(*insn));
 	insn->kaddr = kaddr;
 	insn->end_kaddr = kaddr + buf_len;